Try one of those styrofoam pool noodles…remove the rubber pads from the metal, save them or if you can’t save them buy something similar, like rubber floor protectors for furniture. Cut the noodle in two pieces, slice one side and fit it over the metal pieces. Attach rubber protector to keep it steady and to add height if needed. This is similar to the person who suggested pipe insulation but the styrofoam noodle is firmer.

I took a wooden box for 2 wine bottles. Turned it, glued a peace of wood on the back and secured it with some nails. Painted it, glued some pieces of felt under it and ready.
If it is not high enough, you make your own frame from scraps of wood. And if you want it sloped for any reason,, then you saw the sides in the wished inclination.

Alternatively, get the second panel, then cut a wooden panel, 1.5″ or 1.75″ thick, about 4″ smaller all around (or a rectangular wooden frame the same height – 1x2s will work for this). Paint if desired. Attach with liquid nails or wood glue to the top of the original panel (and/or nails or screws), then sandwich the second panel on top and attach.

GET 4 PIECES OF DOWEL, 2 1/2-4 ” INCHES LONGER THAN THE 4 SUPPORTS. USE A HACKSAW TO CUT THE 4 SUPPORTS, INSTALL THE DOWLING INSIDE THE SUPPORTS. wHEN THE HEIGHT AND THE ANGLE SUITS U, USE CAULK, OR GLUE TO HOLD IN PLACE. HTH

I’m actually considering adding a metal tubing to the joints using supplies I already have from old unusued rods. These can be added to joints and then bent slightly so that it doesn’t add too much width to the bottom of the stool (which could interfere with my chair)

A pool Noodle wrapped around the bars (wear they hit the floor) would raise it about an inch. You’d have to coat the pool noodle with a Car inner tube, or something rubber to provide a non slip surface.

You could also add 4 small legs, one to each of the top corners of the panel, and then add another panel on top of that. Just make sure you leave enough room so that the new panel and the protruding part of the metal frame don’t interfere with each other. If you want it to look original, you could buy a second footstool and use the panel from that as the new top panel.